DESCRIPTION: (Investigator's Abstract): The overall objective of this project is to investigate the role(s) of sulfhydryl (SH) compounds in the cytotoxic effects of selenite. The investigators will determine the sensitivity of drug-resistant tumors to selenite in vivo using three alternative animal models: the growth of subcutaneous human ovarian and breast tumors and intraperitoneal ovarian tumors in athymic (nude) mice. They will also determine whether selenite can prevent the development of drug resistance in vitro in a population of drug-sensitive tumor cells.